There are many examples of radio communications systems in which data is communicated using Orthogonal Frequency Division Multiplexing (OFDM). Television systems which have been arranged to operate in accordance with Digital Video Broadcasting (DVB) standards for example, use OFDM for terrestrial and cable transmissions. OFDM can be generally described as providing K narrow band sub-carriers (where K is an integer) which are modulated in parallel, each sub-carrier communicating a modulated data symbol such as Quadrature Amplitude Modulated (QAM) symbol or Quadrature Phase-shift Keying (QPSK) symbol. The modulation of the sub-carriers is formed in the frequency domain and transformed into the time domain for transmission. Since the data symbols are communicated in parallel on the sub-carriers, the same modulated symbols may be communicated on each sub-carrier for an extended period. The sub-carriers are modulated in parallel contemporaneously, so that in combination the modulated carriers form an OFDM symbol. The OFDM symbol therefore comprises a plurality of sub-carriers each of which has been modulated contemporaneously with different modulation symbols. During transmission, a guard interval filled by a cyclic prefix of the OFDM symbol precedes each OFDM symbol. When present, the guard interval is dimensioned to absorb any echoes of the transmitted signal that may arise from multipath propagation.
As indicated above, the number of narrowband carriers K in an OFDM symbol can be varied depending on operational requirements of a communications system. The guard interval represents overhead and so is preferably minimized as a fraction of the OFDM symbol duration in order to increase spectral efficiency. For a given guard interval fraction, the ability to cope with increased multipath propagation whilst maintaining a given spectral efficiency can be improved by increasing the number K of sub-carriers thereby increasing the duration of the OFDM symbol. However, there can also be a reduction in robustness in the sense that it may be more difficult for a receiver to recover data transmitted using a high number of sub-carriers compared to a smaller number of sub-carriers, because for a fixed transmission bandwidth, increasing the number of sub-carriers K also means reducing the bandwidth of each sub-carrier. A reduction in the separation between sub-carriers can make demodulation of the data from the sub-carriers more difficult for example in the presence of Doppler frequency. That is to say that although a larger number of sub-carriers (high order operating mode) can provide a greater spectral efficiency, for some propagation conditions, a target bit error rate of communicated data may require a higher signal to noise ratio to achieve than required for a lower number of sub-carriers.